Paramagnetism in Metallacarboranes: The Polyhedral Chromadicarbaborane Systems

The chromadicarbaboranes CpCrC₂B_n-3H_n-1 (8 ≤ n ≤ 12) are of interest in providing stable paramagnetic deltahedral metallaboranes among which the 12-vertex CpCrC₂B₉H₁₁ has been synthesized by Hawthorne and co-workers. Density functional theory shows that the lowest-energy such structures are quartet spin-state Cr(III) structures in which the central CrC₂B_n-3 units exhibit most spherical closo deltahedral geometries similar to those found in the borane dianions B_nH_n^2-. Higher-energy doublet CpCrC₂B_n-3H_n-1 (8 ≤ n ≤ 11) structures are found exhibiting central CrC₂B_n-3 isocloso deltahedral geometries, thereby providing a degree 6 vertex for the chromium atom. The lowest-energy CpCrC₂B_n-3H_n-1 (8 ≤ n ≤ 11) structures all have both carbon atoms at degree 4 vertices. However, the lowest-energy CpCrC₂B₉H₁₁ structures all have central CrC₂B₉ icosahedra and thus lack degree 4 vertices for the carbon atoms. For all of the CpCrC₂B_n-3H_n-1 (8 ≤ n ≤ 12) systems the lowest-energy isomers are those with the maximum number of Cr-C edges in contrast to the related CpCoC₂B_n-3H_n-1 systems.